Heat-Transfer Sheet, Heat Transfer System, and Method of Using Heat-Transfer Sheet

ABSTRACT

This invention provides (i) a heat-transfer sheet which comes into close contact with a heat generator and a radiator when caught there between and can be used repeatedly, (ii) a method of using the heat-transfer sheet, and (iii) a heat transfer system comprising the heat-transfer sheet. The heat-transfer sheet is made of expanded graphite and disposed between a heat generator and a radiator, and its bulk density is less than 1.0 Mg/m 3 . Because its bulk density is low, it is easily compressed between the heat generator and the 
     radiator even under low pressure to be put into close contact with them. Thus, the thermal resistance between the heat generator and the heat sink is low and, hence, the heat generator is efficiently cooled down.

TECHNICAL FIELD

This invention relates to a heat-transfer sheet. Radiator such as heat sinks is used to cool heat generators such as CPU's of computers, etc. If a radiator does not come into close contact with a heat generator, thermal conductivity from the heat generator to the radiator is low and the heat generator is not cooled sufficiently. Accordingly, usually disposed between such a heat generator and such a radiator is a sheet which has high thermal conductivity and comes into close contact with the heat generator and the radiator.

This invention relates to (i) a heat-transfer sheet which is caught between a heat generator and a radiator, (ii) a method of using the heat-transfer sheet, and (iii) a heat transfer system comprising the heat-transfer sheet.

BACKGROUND ART

Graphite sheets are usually used as heat-transfer sheets which are caught between such heat generators and radiators. A graphite sheet is caught and compressed between a heat generator and a radiator; therefore, the graphite sheet comes into close contact with the heat generator and the radiator, reducing the thermal resistance of contact surfaces and, thereby, raising the cooling efficiency. The pressure applied to the graphite sheet depends on the force securing the heat sink to the heat generator. If large force is applied to a CPU, it may be deformed. Accordingly, force securing heat sinks to heat generators has been reduced from 5 MPa or so to as small as 2 MPa. If force securing a heat sink to a heat generator is small, the graphite sheet therebetween does not come into close contact with the heat generator or the heat sink, raising the thermal resistance and, thereby, reducing the cooling efficiency.

To solve the above problem, the Japanese Unexamined Patent Publication No. 2004-363432 discloses a heat-transfer sheet comprising (i) a substance which is liquid at normal temperature and whose phase does not change at the temperature range wherein it is used and (ii) a graphite sheet. Because the liquid in the graphite sheet can move freely, the graphite sheet is disposed in minute concave areas, or recesses, and the liquid accumulates in relatively large concave areas of 5-100 μm in the surfaces of the heat generator and the heat sink if the force securing a heat sink to a heat generator is small. Thus, gaps are prevented from being formed between the heat-transfer sheet and the heat generator and between the heat-transfer sheet and the heat sink and, hence, the thermal resistance is minimized and the heat generator is well cooled.

However, because the liquid exists in the graphite sheet, the heat-transfer sheet is not compressed very well, namely, it does not come into close contact with the heat generator or the heat sink; therefore, the thermal resistance is not low enough.

Besides, the production of the heat-transfer sheet requires a step of impregnating the graphite sheet with the liquid. Accordingly, it cannot be produced relatively efficiently, its production cost is relatively high, the liquid may deteriorate, and nearby components may be polluted by liquid coming out of the graphite sheet.

DISCLOSURE OF INVENTION

Problems that the Invention is to Solve

Accordingly, the object of the present invention is to provide (i) a heat-transfer sheet which comes into close contact with a heat generator and a radiator when caught therebetween and can be used repeatedly, (ii) a method of using the heat-transfer sheet, and (iii) a heat transfer system comprising the heat-transfer sheet.

Means of Solving the Problems

According to the first feature of the present invention, there is provided a heat-transfer sheet which is made of expanded graphite and disposed between a heat generator and a radiator and whose bulk density is less than 1.0 Mg/m³.

According to the second feature of the present invention, there is provided the heat-transfer sheet according to the first feature. Its compressibility is 50% or more and its recovery is 5% or more when it is thicknesswise compressed by the pressure of 34.3 MPa.

According to the third feature of the present invention, there is provided a heat transfer system comprising (i) a radiator and (ii) the heat-transfer sheet according to the first or second feature which is disposed between the radiator and a heat generator.

According to the fourth feature of the present invention, there is provided a method of using a heat-transfer sheet which is made of expanded graphite, whose bulk density is less than 1.0 Mg/m³, and which is disposed between a heat generator and a radiator to be thicknesswise compressed therebetween by the pressure of 2.0 MPa or less.

According to the fifth feature of the present invention, there is provided a method of using a heat-transfer sheet which is made of expanded graphite, whose bulk density is less than 0.9 Mg/m³, and which is disposed between a heat generator and a radiator to be thicknesswise compressed therebetween by the pressure of 1.5 MPa or less.

The radiator according to the present invention includes devices with the function of dissipating the heat of heat-transfer sheets to gas, liquid, and other members by radiation, convection, heat transfer, etc., devices with the function of absorbing the heat of heat-transfer sheets, and devices with both the heat-dissipating function and the heat-absorbing function.

EFFECTS OF THE INVENTION

The advantage offered by the first feature of the present invention is as follows. Because the bulk density of the heat-transfer sheet is low, it is easily compressed between a heat generator and a radiator even under relatively low pressure and, hence, comes into close contact with the heat generator and the radiator. Therefore, the thermal resistance between the heat generator and the radiator is small and, hence, the cooling efficiency is high.

The advantage offered by the second feature of the present invention is as follows. Because the recovery of the heat-transfer sheet is high, its bulk density remains below a certain level even after it is used more than once. Therefore, the heat-transfer sheet comes into close contact with a heat generator and a radiator even after it is used more than once, the thermal resistance between the heat generator and the radiator remaining low. Thus, the heat-transfer sheet can be used repeatedly. Because the heat-transfer sheet can be used repeatedly, it contributes toward resources saving.

The advantage offered by the third feature of the present invention is as follows. Because the bulk density of the heat-transfer sheet is low, it is easily compressed between a heat generator and a radiator even under relatively low pressure and, hence, comes into close contact with the heat generator and the radiator. Therefore, the thermal resistance between the heat generator and the radiator is small and, hence, the cooling efficiency is high.

The advantage offered by the fourth feature of the present invention is as follows. Because its bulk density remains below a certain level even after it is used more than once, it comes into close contact with a heat generator and a radiator even after it is used more than once and, hence, the thermal resistance between the heat generator and the radiator remains low. Thus, the heat-transfer sheet can be used repeatedly. Besides, the force securing a radiator to a heat generator, or the pressure applied to a heat-transfer sheet therebetween, can be reduced compared with the pressure which has been applied to heat-transfer sheets so far; therefore, the load stress on the heat generator is reduced and, hence, damage to the heat generator is prevented.

The advantage offered by the fifth feature of the present invention is as follows. The heat-transfer sheet comes into close contact with a heat generator and a radiator and can be used repeatedly.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described below by referring to drawings.

The heat-transfer sheet of the present invention is used to cool such heat generators as CPUs of computers, printed circuit boards of mobile phones, DVD recorders, and servers. The heat-transfer sheet is put between a heat generator and a radiator such as a head sink or a cooling fan. The thermal resistance of the heat-transfer sheet is small when it is caught and compressed between a heat generator and a radiator. Namely, the thermal conductivity of the heat-transfer sheet is high when it is caught and compressed between a heat generator and a radiator.

Thermal resistance is the temperature difference between two points separate from each other of a member which is fed with heat from a heat generator divided by the heat-generation rate of the heat generator. In FIG. 1 (B), the thermal resistance is as follows.

Thermal resistance=(temperature at point “B”−temperature at point “A”)/amount of heat-generation of heat generator

The heat-transfer sheet of the present invention is a sheet of expanded graphite which is made by soaking natural or kish graphite in a liquid such as sulfuric or nitric acid and then heat-treating the graphite over 400° C. Its thickness is 0.05-5.0 mm and its bulk density is less than 1.0 Mg/m³.

The expanded graphite is fibrous or caterpillar-like. The expanded-graphite fibers are, for example, 1.0 mm or so in length and 300 μm or so in diameter and entangled.

The heat-transfer sheet of the present invention may be made of expanded graphite containing a certain amount (for example, 5% or so) of binder such as phenol resin or rubber.

Any processes for making the heat-transfer sheet of the present invention of the above expanded graphite may be adopted.

As the bulk density of expanded-graphite sheets made of such expanded graphite as described above increases, their thermal conductivity of parallel to surface increases and their flexibility decreases. Accordingly, their bulk density is adjusted in accordance with their uses. Expanded-graphite sheets used as heat-transfer sheets are usually given high bulk density (for example, 1.3 Mg/m³ or more) for high thermal conductivity. Expanded-graphite sheets used as heat-insulating sheets and electromagnetic-wave shielding sheets for walls, etc. are usually given low bulk density (for example, 1.0 Mg/m³ or less).

In the case of the heat-transfer sheet of the present invention, more importance is given to flexibility than to thermal conductivity. The heat-transfer sheet of the present invention is an expended-graphite sheet which has bulk density less than 1.0 Mg/m³ and is used as heat-insulating sheets and electromagnetic-wave shielding sheets. If an expanded-graphite sheet is given a bulk density of 1.0 Mg/m³ or more, its flexibility is low and it does not come into close contact with a heat generator or a radiator. Accordingly, the expanded-graphite sheet of the present invention is given a bulk density less than 1.0 Mg/m³ so that it will come into close contact with a heat generator and a radiator as described later. It is preferable particularly if the expanded-graphite sheet is given a bulk density of 0.9 Mg/M³ or less for the reason to be described later.

Next, how to use the heat-transfer sheet of the present invention will be described.

FIG. 1 (A) is an illustration of an example of how to use the heat-transfer sheet of the present invention. FIG. 1 (B) shows temperature-measuring points “A” and “B.” The reference sign “H” is a heat generator such as a CPU of a computer; reference sign 2, a radiator such as a heat sink; reference sign “F.” a cooling fan attached to the heat sink 2.

As shown in FIG. 1, the heat-transfer sheet 1 of this invention is caught between the heat generator “H” and the heat sink 2. When the heat sink 2 is secured to the heat generator “H” with a fixing part “S” such as a clamp, the heat-transfer sheet 1 is squeezed between the heat generator “H” and the heat sink 2.

Because the bulk density of the heat-transfer sheet 1 is less than 1.0 Mg/M³, it is compressed between the heat generator “H” and the heat sink 2 when pressure is applied to it. Accordingly, as the heat-transfer sheet 1 is compressed, it comes into close contact with the heat generator “H” and the heat sink 2. The reason for it is that because the bulk density of the heat-transfer sheet 1 is low and there are spaces between graphite fibers, graphite fibers at the surfaces of the heat-transfer sheet 1 enter concave areas, or recesses, in the surfaces of the heat generator “H” and the heat sink 2.

Accordingly, both the thermal resistance between the heat generator “H” and the heat-transfer sheet 1 and that between the heat-transfer sheet 1 and the heat sink 2 decrease. Because the bulk density of the heat-transfer sheet 1 is less than 1.0 Mg/M³ and its thermal conductivity of perpendicular to surface is about 5 W/m·k, the thermal resistance between the heat generator “H” and the heat sink 2 is small and, thus, the thermal conductivity therebetween is high. Thus, the heat generator “H” is efficiently cooled down by the heat-transfer sheet 1 and the heat sink 2.

Besides, because thermal conductivity of parallel to surface of the heat-transfer sheet 1 is about 50-200 W/m·k which is larger than its thermal conductivity of perpendicular to surface, its temperature distribution of parallel to surface is even; therefore, heat spots do not formed on the surfaces of the heat-transfer sheet 1, heat generator “H,” or heat sink 2.

Moreover, because the heat-transfer sheet 1 is simply caught between the heat generator “H” and the heat sink 2, it can be replaced with a new one 1 easily.

The heat-transfer sheet 1 may be affixed to the heat sink 2 with adhesive or the like so long as the heat-transfer sheet 1 is disposed between the heat generator “H” and the heat sink 2.

The heat-transfer sheet 1 and the heat sink 2 constitute the radiator described in the claims of the present invention.

The heat generator “H” is fitted with the radiator as follows.

The heat-transfer sheet 1 is first put on the heat generator “H” and, then, the heat sink 2 is put on the heat-transfer sheet 1. Next, the heat generator “H.” heat-transfer sheet 1, and heat sink 2 are caught between a clamp “S” and a printed circuit board if the heat generator “H” is a CPU. Thus, the heat generator “H” is fitted with the radiator.

The heat sink 2 may be fitted with a cooling fan “F” for larger cooling capacity, the heat transfer system constituted by the heat-transfer sheet 1, heat sink 2, and cooling fan “F.”

Alternatively, the radiator may be constituted by a heat-transfer sheet 1 and a heat-dissipating component such as a cooling fan “F” or a heat absorbing component such as a cooling-water jacket.

If the heat-transfer sheet 1 designed to have compressibility of 50% or more and recovery of 5% or more when it is thicknesswise compressed by pressure of 34.3 MPa for the first time, its bulk density remains less than 1.0 Mg/m³ after it is compressed more than one time and the last pressure is removed. Accordingly, even after the heat-transfer sheet 1 is used more than one time, it comes into close contact with the heat generator “H” and the heat sink 2 when it is compressed therebetween. Therefore, the thermal resistance between the heat generator “H” and the heat sink 2 remains low even after the heat-transfer sheet 1 is used more than one time. Thus, the heat-transfer sheet 1 can be used repeatedly.

Particularly if the heat-transfer sheet 1 designed to have compressibility of 55% or more and recovery of 6% or more when it is thicknesswise compressed by pressure of 34.3 MPa for the first time, its bulk density remains more certainly less than 1.0 Mg/m³, for example less than 0.9 Mg/m³, after it is compressed more than one time and the last pressure is removed. Accordingly, the heat-transfer sheet 1 can be used more repeatedly.

If the compressibility of the heat-transfer sheet 1 is less than 50% when it is thicknesswise compressed by pressure of 34.3 MPa for the first time, it is not desirable because the heat-transfer sheet 1 does not come into close contact with the heat generator “H” or the heat sink 2. If the recovery of the heat-transfer sheet 1 is less than 5% when it is thicknesswise compressed by pressure of 34.3 MPa for the first time, it is not desirable because the heat-transfer sheet 1 does not come into close contact with the heat generator “H” or the heat sink 2 when it is used for the second time and further.

Even though the bulk density of the heat-transfer sheet 1 is less than 1.0 Mg/m³, the bulk density of the heat-transfer sheet 1 after the removal of the pressure may be 1.0 Mg/m³ or more if the pressure is too large when the heat-transfer sheet 1 and the heat sink 2 is secured to the heat generator “H” by a clamp “S.” If the heat-transfer sheet 1 is compressed by pressure over 2.0 MPa, the heat generator “H” is exposed to large load stress and the repetitive usability of the heat-transfer sheet 1 is reduced.

Therefore, if the heat generator “H” is fitted with the heat-transfer sheet 1 and the heat sink 2 so as to apply pressure of 2.0 MPa or less, preferably 1.5 MPa or less, to the heat-transfer sheet 1, the bulk density of the heat-transfer sheet 1 is less than 1.0 Mg/m³ after the removal of the pressure. Therefore, the heat-transfer sheet 1 can be used repeatedly and no damage is done to the heat generator “H.”

If the heat-transfer sheet 1 is designed to have bulk density 0.9 Mg/m³ or less and the heat generator “H” is fitted with the heat-transfer sheet 1 and the heat sink 2 so as to apply pressure of 1.5 MPa or less to the heat-transfer sheet 1, the bulk density of the heat-transfer sheet 1 is 0.9 Mg/m³ of less after the removal of the pressure. Therefore, the heat-transfer sheet 1 can be used repeatedly and comes into closer contact with the heat generator “H” and the heat sink 2, and the thermal resistance between the heat generator “H” and the heat sink 2 is small.

Particularly if the heat-transfer sheet 1 is designed to have bulk density 0.8 Mg/m³ or less and the heat generator “H” is fitted with the heat-transfer sheet 1 and the heat sink 2 so as to apply pressure of 1.0 MPa or less to the heat-transfer sheet 1, the bulk density of the heat-transfer sheet 1 is 0.8 Mg/M³ of less after the removal of the pressure. Therefore, the heat-transfer sheet 1 comes into even closer contact with the heat generator “H” and the heat sink 2 and has enough recovery.

If the heat-transfer sheet 1 is treated so as to reduce the impurities in it, such as sulfur and iron, to 10 ppm or less, particularly reduce sulfur to 1 ppm or less, components and devices fitted with the heat-transfer sheet 1 are prevented from deteriorating due to impurities.

If film of resin such as polyethylene terephthalate is laminated between the heat-transfer sheet 1 and the heat generator “H” and/or the heat sink 2, fragments of expanded-graphite fibers of the heat-transfer sheet 1 are prevented from scattering. Any resin films may be used so long as their thicknesswise thermal conductivity is about the same as that of the heat-transfer sheet 1 and they are resistant to the temperature of about 100° C.

FIRST EMBODIMENT

The compressibility and recovery of the heat-transfer sheet 1 were measured when it was thicknesswise compressed by pressure of 34.3 MPa.

The bulk density-compressibility relation and the bulk density-recovery relation were ascertained by changing the bulk density of the 0.5-millimeter thick heat-transfer sheet 1 from 0.1 Mg/m³ to 0.5, 0.8, 1.0, 1.2, 1.5 and 1.8 Mg/m³. The compressibility of the heat-transfer sheet 1 was calculated by dividing its thickness under the pressure by its thickness before the compression. The recovery of the heat-transfer sheet 1 was calculated by dividing its thickness after the removal of the pressure by its thickness before the compression.

As shown in FIG. 2 (A), as the bulk density increases, the compressibility decreases and the recovery increases.

By and large, as the compressibility increases, the recovery decreases. In the compressibility range over 50%, the slope of recovery is gentle. Particularly in the compressibility range from 55% to 75%, the recovery changes little.

Thus, if the bulk density of the heat-transfer sheet 1 is such that the compressibility is 50% or more, particularly 55-75%, namely, the bulk density is less than 1.0 Mg/m³ [see FIG. 2 (A)], the compressibility is high and the recovery is kept in a certain range.

SECOND EMBODIMENT

To ascertain the relation between the thermal conductivity of the heat-transfer sheet 1 and the thicknesswise pressure applied to it, it was put between a CPU (Intel's Celeron Processor 2 GHz) and a heat sink (Intel's aluminum heat sink for Celeron) and the CPU was run at a constant processing speed. And the difference between the temperature in the CPU and the temperature of the heat sink was measured.

As shown in FIG. 1 (B), the distance between the measuring point “A” on the heat sink and the measuring point “B” in the CPU was 20 mm.

Four heat-transfer sheets 1 of the same thickness of 0.5 mm and different bulk densities of 0.1, 0.5, 0.8, and 1.0 Mg/m³ were prepared, and the pressure (pressure pressing down the heat sink to the CPU) was varied from 0.1 MPa to 0.5, 1.0, 2.0, and 5.0 MPa.

Small temperature difference means high thermal conductivity, or small thermal resistance, of the heat-transfer sheet 1. Large temperature difference means low thermal conductivity, or large thermal resistance, of the heat-transfer sheet 1.

As shown in FIG. 3, the temperature difference shows a tendency to decrease as the pressure increases in any of the four cases of different bulk densities. It is also shown in FIG. 3 that temperature difference becomes constant when the pressure goes beyond a certain level in any cases. Namely, raising the pressure beyond such a certain level does not reduce the temperature difference any more.

It is also shown in FIG. 3 that as the bulk density lowers, such a certain level lowers and that temperature difference is almost constant in the pressure range over 2.0 MPa in any cases of different bulk densities.

It is further shown in FIG. 3 that temperature difference is almost constant in the pressure range over 1.0 MPa, particularly in the range over 1.5 MPa, if the bulk density is 0.8 Mg/m³ or less.

THIRD EMBODIMENT

The relation between the bulk density and the thermal conductivity of the heat-transfer sheet 1 of the present invention under a certain level of pressure was ascertained. FIG. 1 (B) shows the temperature measuring points “A” on a heat think and “B” in a CPU, the distance between the two points being 20 mm.

Seven 0.5-mm thick heat-transfer sheets 1 were prepared so that their respective bulk densities would be 0.1, 0.5, 0.8, 1.0, 1.2, 1.8, and 2.0 Mg/M³ when they were put between the CPU and the heat sink and pressure of 1.0 MPa was applied to them. Under the conditions, the temperature difference between the two measuring points “A” and “B” was measured.

As shown in FIG. 4 (A), as the bulk density increases from 0.1 Mg/m³ to 0.8 Mg/m³, the temperature difference increases very little, but as the bulk density increases from 0.8 Mg/m³ to 1.0 Mg/m³, the temperature difference increases very much. Namely, it is shown in FIG. 3 that if the pressure is about 1.0 MPa, the thermal conductivity of the heat-transfer sheet 1 changes greatly in the bulk-density range from 0.8 to 1.0 Mg/m³.

The comparison between the result of this embodiment and that of the second embodiment suggests that the bulk-density range where the thermal conductivity changes greatly lowers if the pressure lowers below 1.0 MPa, while the range does not change very much if the pressure rises beyond 1.0 MPa.

FOURTH EMBODIMENT

It was ascertained whether the relation between the bulk density and the thermal conductivity of the heat-transfer sheet 1 of the present invention changed or not depending on the number of times of compression under constant pressure. FIG. 1 (B) shows the temperature measuring points “A” on a heat think and “B” in a CPU, the distance between the two points being 20 mm.

Four heat-transfer sheets 1 of the same thickness of 0.5 mm and different bulk densities of 0.1, 0.5, 0.8, and 1.0 Mg/m³ were prepared. Each heat-transfer sheet 1 was compressed between the CPU and the heat sink four times.

As shown in FIG. 4 (B), the temperature difference is constant regardless of the number of times of compression in any case of the four heat-transfer sheets 1 of different bulk densities. Thus, it was ascertained that the thermal conductivity of the heat-transfer sheet 1 was determined by (i) its original bulk density before it was first caught between a CPU and a heat sink and (ii) the pressure applied to it therebetween.

INDUSTRIAL APPLICABILITY

The heat-transfer sheet of the present invention is suitable for use as a sheet to transfer heat from CPU's of computers, mobile phone, etc., DVD recorders, and so on to radiator such as heat sinks and cooling fans.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 (A) is an illustration of an example of how to use the heat-transfer sheet of the present invention. FIG. 1 (B) shows temperature-measuring points “A” and “B.”

FIG. 2 (A) shows the relations between the bulk density and the compressibility/recovery of the heat-transfer sheet 1 of FIG. 1 when it was thicknesswise compressed by the pressure of 34.3 MPa.

FIG. 2 (B) shows the relation between the compressibility and the recovery.

FIG. 3 shows the relation between the pressure thicknesswise applied to the heat-transfer sheet of FIG. 1 and its thermal conductivity. The bulk density of the heat-transfer sheet was varied.

FIG. 4 (A) shows the relation between the bulk density and the thermal conductivity of the heat-transfer sheet of FIG. 1 under constant pressure. FIG. 4 (B) shows the relation between the number of times of compression and the thermal conductivity.

EXPLANATIONS OF LETTERS OR NUMERALS

1 heat-transfer sheet

2 heat sink

H heat generator 

1. A heat-transfer sheet which is made of expanded graphite and disposed between a heat generator and a radiator and whose bulk density is less than 1.0 Mg/m³.
 2. The heat-transfer sheet according to claim 1 whose compressibility is 50% or more and whose recovery is 5% or more when it is thicknesswise compressed by the pressure of 34.3 MPa.
 3. A heat transfer system comprising a radiator and the heat-transfer sheet of claim 1 disposed between the radiator and a heat generator.
 4. A method of using a heat-transfer sheet which is made of expanded graphite, whose bulk density is less than 1.0 Mg/m³, and which is disposed between a heat generator and a radiator to be thicknesswise compressed therebetween by the pressure of 2.0 MPa or less.
 5. A method of using a heat-transfer sheet which is made of expanded graphite, whose bulk density is less than 0.9 Mg/m³, and which is disposed between a heat generator and a radiator to be thicknesswise compressed therebetween by the pressure of 1.5 MPa or less.
 6. A heat transfer system comprising a radiator and the heat-transfer sheet of claim 2 disposed between the radiator and a heat generator. 